Enhanced dispersion and material properties of multi-walled carbon nanotube composites through turbulent Taylor-Couette flow

Enhanced dispersion and material properties of multi-walled carbon nanotube composites through turbulent Taylor-Couette flow

Composites: Part A 95 (2017) 118–124 Contents lists available at ScienceDirect Composites: Part A journal homepage: www.elsevier.com/locate/composit...

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Composites: Part A 95 (2017) 118–124

Contents lists available at ScienceDirect

Composites: Part A journal homepage: www.elsevier.com/locate/compositesa

Enhanced dispersion and material properties of multi-walled carbon nanotube composites through turbulent Taylor-Couette flow Sang-Eui Lee b, Sung-Hoon Park a,⇑ a b

Department of Mechanical Engineering, Soongsil University, 369 Sangdo-ro, Donjak-gu, Seoul 156-743, South Korea Samsung Electronics, 130 Samsung-ro, Suwon 446-712, South Korea

a r t i c l e

i n f o

Article history: Received 19 September 2016 Received in revised form 3 January 2017 Accepted 7 January 2017 Available online 8 January 2017 Keywords: A. Carbon nanotube B. Electrical conductivity C. Dispersion D. Turbulent Taylor-Couette flow

a b s t r a c t We report enhanced dispersion conditions and electrical properties of multi-walled carbon nanotube (MWCNT) composites through the use of turbulent Taylor-Couette flow. The vortex flow, which is created in a cavity between concentric inner rotating and outer stationary cylinders, provided a uniform dispersion of MWCNTs in a polymer matrix through debundling highly entangled carbon nanotubes. Compared with a three-roll milling process that can apply mechanical shear forces to bundles of MWCNTs, the turbulent Taylor-Couette process generates fluidic shear forces that can more effectively exfoliate MWCNTs, particularly for high MWCNT concentrations. This was validated by the high electrical conductivity that reached 1640 S/m for uniformly dispersed carbon nanotubes in a silicone polymer matrix (at 21.8 Vol% of MWCNT). In view of their high electrical conductivity and uniform dispersion, the MWCNT composites can be promising for rapid electric heating elements. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Polymer composites incorporating conducting fillers, such as carbon fiber, nanocarbon, carbon black, and metal particle, have been investigated for various applications, including heating elements, electromagnetic interference (EMI) shielding, electronic packaging, radar absorption, and structural reinforcement [1–9]. Among them, heat-related applications, such as temperature sensors, patternable micro heaters, flexible deicing units, thermoelectric devices, and heating glasses for vehicles, are some of the most practical choices of the material [7–13]. Through electric resistive heating of conducting composites, electrical energy can be easily and quantitatively transferred into heat. The properties of the conductive polymer composites depend on the types of filler and matrix, filler geometry (thickness or diameter, length, and dimensionality), surface chemistry of the filler, and dispersion and processing method determining filler distribution, such as alignment and interlocking [11,14–16]. A variety of nanocarbon fillers, including carbon nanotubes (CNTs) (single-walled CNT (SWCNT), and multi-walled CNT (MWCNT)), two-dimensional graphene-based nanocarbon (graphene, graphene oxide, graphite nanoplatelets), carbon nanofiber, and their combinations have been used [14– 20]. Among them, MWCNTs can be ideal fillers in terms of their ⇑ Corresponding author. E-mail address: [email protected] (S.-H. Park). http://dx.doi.org/10.1016/j.compositesa.2017.01.005 1359-835X/Ó 2017 Elsevier Ltd. All rights reserved.

high aspect ratios and relatively longer filler distances [17,18], leading to an extremely low electrical percolation threshold (0.06 Vol%), and rapid heating [21]. However, there are still several issues to commercialize the CNT-based composites, such as the intrinsic heavy bundling and aggregation of CNTs, which causes poor reproducibility of their electrical and mechanical properties [16]. Various dispersion methods have also been studied, and can be divided in terms of the driving force for dispersion, and these include ultrasound [22], mechanical shear [21,23], and fluidic shear [24–32]. First, the ultrasonication method is most widely used to exfoliate CNTs. However, this process could easily damage or break the nanotube structures [22]. Furthermore, it is not appropriate in the case of high CNT loading conditions. In mechanical shear processes, such as three-roll milling (TRM) [13,16,21] and dry extrusion [23], strong mechanical shear forces are created between rolls or extruders rotating in opposite directions, which disentangle bundles of multi-walled carbon nanotubes. However, the minimum gap between rolls is a few microns, which could be sub-optimal for nano-size fillers. Fluidic shear methods have also been studied [24–32]. The high-pressure homogenizer, in which a particle-dispersed solution is injected into a specialized chamber where high fluidic shear forces are generated by synergy of the injection pressure and the inner geometry, is used to disentangle and exfoliate cellulose fibrils [24] and CNTs [25,26]. Taylor-Couette (TC) flow and its derivatives have also been studied

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for dispersion purposes [27–32]. The Taylor-Couette flow occurs in a gap between two concentric cylinders, where the cylinders can rotate independently, in terms of rotating direction and speed [27,28]. Our work corresponds to the combination of an inner rotating cylinder and an outer stationary one (xo = 0). For low angular velocities, the flow is stable and purely azimuthal. When the Taylor number (Ta) exceeds a critical value (Tac), the flow becomes unstable, and pairs of counter-rotating vortices are imposed in the azimuthal flow. As Ta increases, the flow evolves to become more complex, as shown in various descriptive words, i.e., wavy, turbulent, strongly turbulent, or featureless turbulent [27]. Various kinds of Taylor vortices or derivatives were reported to be facile and efficient methodology to exfoliate graphites to graphenes with high yield rate [29], and to fragmentize microcellulose diameters into nano-scale cellulose ones [30]; as well as impurity extraction, and liquid–liquid dispersion [31,32]. The aim of this work is to utilize turbulent Taylor-Couette (TTC) flow to enhance the dispersion conditions and electrical properties of MWCNT composites. The turbulent vortex between concentric inner rotating and outer stationary cylinders creates fluidic shear forces to exfoliate heavily bundled MWCNTs. We optimized the TTC flow by controlling the time and rotating speed. Furthermore, we compared this method to the three-roll milling process representing a mechanical force-driven dispersion method, in terms of the electrical conductivity and Joule heating characteristic, which visualizes the degree of dispersion, and the advantage of higher conductivity.

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shear on the dispersion of MWCNTs. The horizontal cavity between the two concentric cylinders is a mixing zone where Taylor vortex flow occurs. The speed of the inner rotating cylinder, vi, can be controlled up to 50 m/s (angular velocity xi = 1923 rad/s), which corresponds to an apparent shear rate of 12,500/s for the reaction gap of 4 mm (d = ro  ri) between the two concentric cylinders. The velocity and angular velocity of the outer vessel are zero (vo = xo = 0). TX-100, corresponding to 5 wt% (wt%) of MWCNTs, was dissolved in 40 ml chloroform by a magnetic stirrer for 5 min, and then certain amounts of MWCNTs were mixed in the solution for 5 min, and then PDMS resin (3.64 g) and curing agent (0.36 g) of mass ratio of 10:1 were added into the mixture of MWCNTs and TX100 solution. The volume fraction (Vol%) of MWCNTs to the PDMS polymer was controlled to be in a range up to 21.8%. The concentration was calculated from the density of MWCNTs (1.85 g/cm3), TX-100 (1.07 g/cm3), and PDMS (1.03 g/cm3). The premixed MWCNT-PDMS solution was homogenized in the Taylor-Couette reactor for given dispersion times and rotating speeds. The dispersion times were scanned up to 10 min, and the speed of the rotator was controlled at 20 and 40 m/s. The temperature of the reactor was maintained below 50 °C by cooling through water circulation in the outer cylinder. After the Taylor vortex flow mixing, the solvent evaporation by vacuuming was made at 25 °C, and then a hot press (Carver) was used for curing to press the MWCNT pastes into 0.5 mm thickness at 130 °C for 30 min. 2.3. Measurement

2. Experimental 2.1. Materials For fabrication of the CNT composites, MWCNTs having 10– 20 nm outer diameter and 50–200 lm length were purchased from Hanwha Chemicals, while Polydimethylsiloxane (PDMS, Sylgard 184) as an elastomeric polymer was purchased from DOW Corning. Triton X-100 (TX-100, polyethylene glycol) for surfactant and chloroform were purchased from Sigma Aldrich. 2.2. Composite fabrication with turbulent TC flow A Filmics nanomixer (model 56-50, Primix Co, Japan) was used as a Taylor-Couette chamber to debundle MWCNTs by fluidic shear force [30]. Fig. 1 shows that the fluidic shear reactor mainly consists of an inner rotating cylinder and an outer stationary one. The right part of the figure shows the anticipated effect of the fluid

ro ri

ωi

ωo=0

The morphology of the raw MWCNTs and MWCNT composites was characterized by scanning electron microscopy (SEM, Phillips XL30) and transmission electron microscopy (TEM, JEOL JEM2010). To measure the dielectric current conductivity (rDC), MWCNT composites were treated with oxygen plasma (Oxford Plasmalab), to minimize electrical contact resistance. Then, gold was sputtered on at a thickness of 50 nm to create contacts. The four-point resistance method was used to measure the resistance (R) for composites with R < 1 G X. A pico-ammeter (Keithley 487) and a source-meter (Keithley 2400) were used for the tests. For rDC measurements, the cross-section of the composite sample was 10 mm wide  0.5 mm thick. The outer current parts were separated by 25 mm, while the inner voltage parts were separated by 15 mm. To evaluate the electric heating properties of MWCNT composite, direct electric current was applied to samples using NI instrument (National Instruments, NI PXI). Thermal images were then recorded by a thermal infrared camera (IR) (Advanced Thermo, TVS-500). MWCNT-PDMS composites were cut into 10  30 mm2 rectangles of 0.5 mm thickness for the measurement. A thermocouple was also connected at both sides of the composite surface during heating, and it was compared with and calibrated the temperature data from the IR camera. A conductive copper tape was used as electrode, and a conductive silver paste was applied between the sample and the electrode, to ensure stable electrical contact. The heating performance of the composites was evaluated by measuring changes in temperature under a constant electric voltage.

3. Results and discussion 3.1. Turbulent Taylor-Couette flow for MWCNT dispersion Fig. 1. Schematic of turbulent Taylor-Couette reactor. The turbulent Taylor-Couette flow-driven shear forces exfoliate heavily entangled MWCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The stability of Taylor-Couette flow strongly depends on the rotor speed, which is described in the Taylor number. The Taylor vortex flow is created when the number exceeds a critical value (Tac) of around 1700, where the Taylor instability occurs [25–27].

Ta ¼

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x2i ri ðri  ro Þ3 Re2i d ¼ ri m2

ð1Þ

xi r i d v i d ¼ Rei ¼ m m

ð2Þ

where Rei is the Reynolds number at the surface of the inner cylinder, m is the kinematic viscosity of the MWCNT-dispersed solution, xi is the angular velocity of the inner cylinder, ri is the outer radius of the inner cylinder, ro is the inner radius of the vessel, and d is the reaction gap (d = ro  ri) between the two concentric cylinders. The Taylor numbers calculated from Eq. (1) for vi of 20 and 40 m/s are 6.8  106 and 2.7  107, respectively, which are much higher than Tac. Moreover, Andereck [25] shows the Taylor vortex flow occurs in Rei above 1800 (Reic). Rei corresponding to the Taylor numbers for the inner rotating speeds of 20 and 40 m/s are 6.7  103 and 1.3  104, respectively. Therefore, Ta in this study is enough for the Taylor vortex flow to be in turbulent form, providing high fluidic shear forces to debundle highly entangled MWCNTs in the polymer matrix.

Electrical conductivity (S/m)

120

1400 1200 1000

Electrical conductivity (S/m)

600 400

0

2

4

6

8

10

Dispersion time (min.) Fig. 3. Dependence of electrical conductivity of 15.7 Vol% MWCNT/PDMS composite on the dispersion time and rotor speed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1.0

IG /ID

0.8

Intensity 0 min 1 min 10 min

0.6 0.4 0.2 0.0 1000

1200

1400

1600

1800

-1

Raman shift (cm ) 0.95

IG /ID

0.90 0.85 0.80 0.75 0.70

1200

Rotator speed 20 m/s 40 m/s

200 0

3.2. Effect of processing parameters of the Taylor-Couette flow To optimize the fluidic milling process while ensuring uniform dispersion for reproducibility, and to minimize any length reduction or damage of MWCNTs, we evaluated the influences of the processing parameters on the DC electrical conductivity of the composite: the dispersion time and rotator speed, as well as the amount of surfactant. For this purpose, the volume fraction of MWCNTs was fixed at 15.7 Vol%. The amount of surfactant was firstly determined for further evaluation of the processing parameters, for which the dispersion time was set to be 2 min. For both the rotator speeds of 20 and 40 m/s, rDC was the highest at 5 wt% of TX-100, as Fig. 2 shows. This can be understood by the general phenomenon for surfactant usage that van der Waals (vdW)-induced aggregation at low surfactant concentration and depletion-induced aggregation at high surfactant concentration define the limits of an intermediate range of usage for evenly-dispersed nanotubes [33]. Even though the amount of the surfactant can be further optimized by scanning the concentration near 5 wt%, the amount of surfactant was fixed at the weight percent for all the other MWCNT concentrations. For the given amount of surfactant, the conductivity of the composites was evaluated with increasing dispersion time and rotor speed. Fig. 3 shows the relationship of the conductivity with varying process parameters. For each rotor speed case, rDC was satu-

800

0

2

4

6

8

10

Dispersion time (min.)

1000

Fig. 4. (a) Raman spectra of 15.7 Vol% MWCNT/PDMS composites at a wavelength of 514.5 nm to determine changes in the D-band and G-band after the turbulent TC milling process (with vi = 40 m/s), and (b) ratio of the intensities of the G-band and D-band peaks (IG/ID). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

800 600 Rotor speed 40 m/s 20 m/s

400 0

2

4

5

6

8

10

12

Surfactant concentration (wt%) Fig. 2. Electrical conductivity of MWCNT/PDMS composites as a function of surfactant concentration with different rotor speeds. (MWCNT concentration = 15.7 Vol%, dispersion time = 2 min.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

rated after 3 min as the dispersion time increased. In addition, the CNT composites for the higher rotor speed (40 m/s) with relatively short dispersion times have higher electrical conductivity than those for 20 m/s. Consequently, one minute of fluidic vortex milling with 40 m/s rotor speed has the highest conductivity (1200 S/m), indicating that the rotor speed is a dominant parameter to debundle MWCNT agglomerates. The saturated/decreased conductivities with increasing processing time (for both 20 and 40 m/s cases) are attributed to

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shortening of the MWCNTs. Fig. 4 shows the Raman spectroscopy of 15.7 Vol% MWCNT composite with varying the turbulent vortex milling time. Raman spectra of the MWCNT composite excited at a wavelength of 514.5 nm were obtained to determine changes in the D band (defect and disorder), and the G band (carbon structure). Fig. 4(a) shows that there was no particular change in the D-band and G-band peaks of the CNTs for all samples. Fig. 4(b) shows that the ratio of the intensities of the D-band and G-band peaks was also constant, e.g., IG/ID = 0.82 for the as-prepared composite, and IG/ID = 0.8 for the 10 min processed composite. While there are shortenings of MWCNTs at the existing defect sites of MWCNTs during the turbulent vortex milling process, this figure indicates that there was no significant disruption in the basic morphology of the carbon atom arrangement in MWCNTs. The trends are quite similar to those of the TRM process [13,16,21].

3.3. Comparison of the dispersion method in high MWCNT loading (TTC milling vs TRM)

Electrical conductivity (S/m)

To further investigate the advantage of turbulent vortex milling in MWCNT dispersion, the turbulent TC flow method was compared with the TRM method, which can be representative of the use of mechanical shear forces, and is a practical processing candidate for mass production. In previous works, we demonstrated a uniform dispersion condition of MWCNTs in a polymer matrix and high electrical conductivity through the TRM process [13,16,21]. The TRM process was set up as follows; The mixing ratios of the MWCNTs and the PDMS polymer were as described as in Vol% without any other additives; The roll diameter was 20 cm; The three-roll speed ratio was 1:2.6:6.8 in the order of feed, middle, and apron rolls, and the rotating frequency of the feed roll was 40 Hz; The pressure between the rolls determining the roll gap was 1.0 MPa. The optimal TRM conditions were found to be 7 passes by monitoring the change of conductivity and error bar as the number of roll passed increased [16,21]. Therefore, optimal process conditions (40 m/s & 1 min for TTC flow, and 7 passes for TRM) were applied for precise comparison. Fig. 5 shows that the electrical conductivities of MWCNT composites were measured with varying MWCNT contents for both processes. A higher conductivity (1640 S/m) was obtained through the turbulent TC milling at 21.8 Vol% MWCNT concentration, while the conductivity of the MWCNT/PDMS prepared by TRM process was 1003 S/m. In addition, the tendency of the higher conductivity obtained by TTC flow was also observed in low MWCNT concentra-

2000 Composite by turbulent TC flow Composite by three-roll milling 1500

1000

500

0 0

5

10

15

20

25

MWCNT concentration (Vol%) Fig. 5. Electrical conductivity of composites prepared by turbulent TC flow and the TRM process as a function of MWCNT concentration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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tions of 1.0 Vol% and 3.8 Vol%, and thus Fig. 5 shows that the difference between the electrical conductivity for the fluidic sheardriven and the mechanical shear-driven processes increased with increasing MWCNT content. Therefore, the comparison shows that the fluidic milling process can be an effective fabrication method for high contents of MWCNTs, as well as low levels of filler concentration. The better dispersion and debundling may be attributed to enhancement in both the electrical conductivity in Fig. 5 and mechanical properties in Fig. S1 and Table S1. The tensile moduli were 73.5 MPa and 41.2 MPa, and the tensile strengths were 8.4 MPa and 7.0 MPa for MWCNT (15.7 Vol%)/PDMS composites fabricated by TTC flow and TRM process, respectively. The electrical conductivity and the tensile modulus were enhanced 71% (= (1139  746)/746) and 78% (=(73.5  41.2)/41/2) for the comparison, respectively. Fig. 6 shows SEM images of MWCNT bundles and MWCNT/ PDMS composites. The figure shows the MWCNTs are highly entangled, and well-aligned to the main axis of the bundles. Fig. 6(a) inset shows a number of walls of 8–15 nm total thickness. Fig. 6(b) and (c) are the SEM images of 21.8 Vol% and 15.7 Vol% MWCNT composites. In spite of the high particle loading, the nanotubes are observed to be evenly dispersed all over the polymer matrix. Fig. 6(d) shows the nanocomposite with 1.0 Vol% MWCNT loading with the fluidic shear process. A good dispersion state of MWCNTs was also clearly evident at this low concentration. This may be attributed to the turbulent TC flow with high Ta (>106) that exceeded Tac (1700). For further investigation, Fig. 7 shows a schematic of the TC flow in the reactor. The good efficiency of debundling and dispersion of the turbulent Taylor vortex flow may result from the nature of the turbulent flow. The moment of inertia of the bundled MWCNTs in the longitudinal direction makes the bundles rotate into the flow direction in the flow. However, Fig. 7 shows that the turbulent vortices overcome the moment of inertia to orient them in a certain direction where the debundling force can be a minimum. The van der Waals forces between two crossed MWCNTs depend on the angles (c) between pairs of tubes, and the diameter and the wall numbers of carbon nanotubes [16,34,35]. As the angle decreases from 0° (totally-aligned state) to 90° (normally-crossed state), the vdW forces decrease. On the other hand, as the diameters and the number of walls increase, the vdW forces increase [34,35]. For the MWCNTs used in this study, having on average 11 walls and 20 nm diameters, the vdW forces vary from 43.4 nN (c = 20°) to 20 nN (c = 90°) [16], which should be overcome for the dispersion and formation of conductive networks. Fig. 7 shows the proposed conceptual model for the effect of the turbulent flow on the debundling. The contact points of pairs of MWCNTs in a nanotube entanglement exhibit a variety of angles. The turbulent flow makes the bundled MWCNTs rotate in the best direction, in which vortex-induced shear forces are sufficient to overcome the vdW forces and to debundle the entangled MWCNTs. In order to evaluate the degree of dispersion of MWCNTs in the polymer matrix, and to demonstrate the heating performance for heat-related applications, the electrical heating characteristics of 21.8 Vol% CNT/PDMS composites prepared by the TTC flow and the TRM process were measured under an applied DC voltage of 5 V, as Fig. 8 shows. Fig. 8(a) shows a steady-state temperature distribution over the composite with 21. 8 Vol% MWCNTs prepared by TTC flow. The temperature was uniform and symmetric for the center of the sample. The temperature uniformity is determined by the thickness uniformity and the degree of particle dispersion. The thickness deviation was ±20 lm for the average thickness of 500 lm over the composite. Therefore, the particle dispersion in the matrix may be estimated to be even and uniform, which is also supported

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(b)

(a)

10 nm

3 µm

(c)

1 µm

(d)

1 µm

1 µm

Fig. 6. SEM images of (a) highly entangled raw CNT bundles, (b), (c), and (d) uniformly dispersed MWCNT/PDMS composites with the MWCNT concentrations of 21.8, 15.7, and 1.0 Vol%. The inset figure of (a) is TEM imagery of MWCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. The proposed conceptual model for turbulent TC flow-driven de-bundling of MWCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

by the observation of MWCNTs distribution in the SEM images of Fig. 6. Fig. 8(b) displays the temperature measured at the center of the composites with increasing time. The composite of the TTC flow possessed a higher steady-state temperature of 192 °C, while the composite by the TRM process reached 124 °C. This can result from the difference in the electrical resistance (R) at the steady state, 2.2 X for TTC flow and 3.4 X for the TRM process, because the electrical energy (Ee) applied to the material has a relationship with the stored thermal energy (ET), Ee = P  time = (V2/R) 

time  C  m  DT = ET, where P is the applied power, and C and m are the specific heat and the mass of the composite, respectively. DT is the final temperature (Tf) minus the initial temperature (Ti, 25 °C). Therefore, R is inversely proportional to Tf, as C  m can be taken as a constant for the two types of the composites for the same MWCNT concentration. This indicates that an effective dispersion and debundling of MWCNTs resulted in higher conductivity, finally leading to a high steady-state temperature under a constant voltage level, and also a faster heating speed at the early stage of Joule heating, as Fig. 8(b) shows.

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(a)

Appendix A. Supplementary material Tensile stress-strain curve (Fig. S1) and tensile properties (Table S1) of MWCNT(15.7 Vol%)/PDMS composite fabricated by three-roll milling process and turbulent Taylor-Couette flow. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.compositesa. 2017.01.005.

°

References

° °

(b)

210 180

Temperature (°C)

123

150 120 90 60

Composite by turbulent TC flow Composite by three-roll milling

30 0

10

20

30

40

50

60

70

Time (sec) Fig. 8. Electric heating properties of MWCNT/PDMS composites under an applied DC voltage: (a) steady-state temperature distribution of the composite prepared by turbulent TC flow, and (b) change in surface temperature of the composites prepared by TTC flow and TRM processes with increasing time. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Conclusion In this paper, we report enhanced dispersion conditions and electrical properties of MWCNT composites through the use of turbulent Taylor-Couette flow. The turbulent vortex flow that is created between a rotating cylinder and a stationary one produced efficient dispersion and debundling of highly entangled MWCNTs, without significant structural damage. Compared with the threeroll milling method, the fluidic shear-force-driven turbulent flow was demonstrated to be optimal in creating a high level of electrical conductivity, especially in the case of high filler concentrations. In addition, the fabricated MWCNT/PDMS composite was also demonstrated to be a promising material for rapid electric heating elements, owing to the high electrical conductivity and uniform dispersion. Acknowledgment This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2016R1C1B1012710).

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